Fix the handbook URL in the maintainer README

Use the canonical documentation address (as in the AppStream file).

doc/developer/MAINTAINER.README

@@ -46,7 +46,7 @@ Links
     2. An attempt to get more interest in Konsole
         https://userbase.kde.org/KonsoleDevelopment
     3. HTML version of Handbook
-        https://docs.kde.org/trunk5/en/applications/konsole/index.html
+        https://docs.kde.org/?application=konsole
     4. KDE schedule https://community.kde.org/Schedules